In modern communications systems, the information included in an error correction coded digital signal may be a digitized voice signal at one point in time and a digital data signal such as computer data, signalling information, digitized facsimile or the like during another time period. The transmission of different types of information or traffic often exists, for example, in cellular telephone systems. In general, these different types of traffic signals are usually coded with error-correction codes whose performance is tailored to the degree of error protection required by a particular type of information. For example, digitized speech may be transmitted using a code that provides 2:1 redundancy of the transmitted bits, while digital data information may require a code providing 4:1 redundancy. This difference in redundancy is based on the relative seriousness of uncorrected errors in a particular type of traffic signal. In this example, because a speech signal can still be interpreted by the human listener despite some transmission errors, a lower redundancy is acceptable. By comparison, digital data tolerates few errors and requires higher redundancy.
In many communications systems such as tactical radio systems, cellular radio telephone systems, or computer data systems provided with the aid of telephone modems, a user may desire to mix alternately data and speech. To accomplish this information mixture, a control voice/data switching signal of some sort is transmitted from the transmitter to the receiver to indicate the type of traffic signals to be expected. That signal, often termed a "voice/data flag", must be interpreted with a high degree of certainty so that digital speech is not passed erroneously to a device expecting digital data. Of course, such errors could cause disruption to the data device as well as loss of a portion of the speech. Data that is interpreted incorrectly as speech could cause unpleasant noises in voice conversations as well as a loss of the intended data. To insure the voice/data switching signal is accurately received, communications systems include with the switching signal a high degree of redundancy. The resulting overhead reduces the communications system capacity.
Accordingly, it would be desirable to have a reliable means to distinguish between, for example, coded speech and data signals, without the overhead of an explicit voice/data switching signal.